Radar sensors are used in motor vehicles, for example, in order to measure the distances, relative speeds, and azimuth angles of vehicles or other objects located in the area in front of the own vehicle. Multiple antenna elements are then disposed, for example, at a distance from one another on a horizontal line, so that different azimuth angles of the located objects result in differences in the path lengths that the radar signals have traveled from the object to the respective antenna element. These path length differences result in corresponding differences in the amplitude and phase of the signals that are received by the antenna elements and evaluated in the pertinent evaluation channels. The angle of incidence of the radar signal, and thus the azimuth angle of the located object, can then be determined by comparing the (complex) amplitudes received in the various channels with corresponding amplitudes in an antenna diagram. The elevation angle of an object can also correspondingly be estimated using antenna elements disposed vertically above one another.
For a single target, the comparison between the received amplitudes and the amplitudes in the antenna diagram can be made by calculating, for each angle in the antenna diagram, a correlation between the vector of the measured amplitudes (for k evaluation channels, this is a vector having k complex components) and the corresponding vector in the antenna diagram. This correlation can be expressed by a so-called deterministic maximum likelihood (DML) function which, when a specific vector of measured amplitudes is given, indicates for each angle the probability that the object is located at that angle. Angle estimation then consists in finding the maximum of this DML function. In this case the DML function is dependent on only a single variable, namely the relevant azimuth angle or elevation angle. The search for the maximum therefore occurs in a one-dimensional search space.
If the radar sensor is locating multiple targets simultaneously, those targets normally differ in terms of their distance and/or relative speed, so that the targets can be separated from one another and the angle estimate can then be performed individually for each target. If the distances and relative speeds of the two targets are so similar to one that a separation is not possible given the limited resolution of the radar sensor, however, the two targets then appear as a single target, and the above-described angle estimate would yield only a single angle as a result. But because there are in fact two targets, interference occurs between the signals that are backscattered from the two targets and become superimposed at the radar sensor. The consequence of this is that the pattern of the received amplitudes no longer corresponds to the antenna diagram for a single target.
It is nevertheless possible to generalize the above-described method for angle estimation to two-target or multi-target estimates. The DML function is then a function of multiple variables, namely of the angles of the various targets, and in the context of an n-target estimate the search space consequently has n dimensions. The location of the maximum of the DML function in this search space then has n components, which indicate the location angles of all n targets.
Multi-target estimation has the disadvantage, however, that searching in a multi-dimensional search space is extremely calculation-intensive. The method is moreover susceptible to errors due to the unavoidable signal noise.
In practical utilization of a radar sensor in motor vehicles, even when only a single radar target is present it often happen that as a result of reflections of the backscattered signal from the road surface or from a guardrail, additional apparent targets, which are in reality merely mirror images of the located object, are simulated. In this case the distances and relative speeds are almost identical. Although the signal reflected from the road surface or guardrail takes a certain detour, it is hardly measurable given the almost raking reflection. It is thus not possible to differentiate between the real target and the apparent target, so that strictly speaking a multi-target estimate would need to be carried out for estimates of the elevation angle when reflections from the road surface are to be expected, and a multi-target estimate would need to be carried out for estimates of the azimuth angle when reflections from a guardrail or a similarly elongated object are to be expected.